Magnetic resonance imaging (MRI) is a well-established medical imaging and diagnostic tool. A great deal of current activity and research relates to interventional and/or intraoperative procedures conducted under MRI guidance (iMRI). For example, in many interventional and intraoperative procedures under MRI guidance, surgical devices such as long needles, guidewires, and catheters are used and it is advantageous for a surgeon to be able to image the local tissue and locate such instruments in conjunction with the magnetic resonance image. To achieve active device profiling during real time MRI, interventional devices have been provided with a radio frequency (RF) antenna, more particularly an RF coil, in the device.
The RF or receive coil is typically located at the distal end of the catheter or other device and receives a signal from excited protons of blood or tissue of its vicinity when they return to equilibrium. The RF coil then sends an electrical signal directly to the MRI scanner by way of an attached coaxial cable. The coaxial cable is typically a very thin coaxial cable that runs through a lumen in the catheter. The presence of long conductive objects, such as coaxial cables, can lead to heating in such RF coils. Medical studies indicate that this effect is due to coupling of the RF field from the MRI system, primarily to the long cable (“Reduction of Resonant RF Heating in Intravascular Catheters Using Coaxial Chokes”, Mark E. Ladd et al., Magnetic Resonance in Medicine 43:61-5-619 (2000); “RF Safety of Wires in Interventional MRI: Using a Safety Index”, Christopher J. YEUNG et al., Magnetic Resonance in Medicine 47:187-193 (2002); “RF Heating Due to Conductive Wires During MRI Depends on the Phase Distribution of the Transmit Field”, Christopher J. YEUNG et al., Magnetic Resonance in Medicine 48:1096-1098 (2002); and “Safety of MRI-Guided Endovascular Guidewire Applications”, Chia-Ying LIU et al. Journal of Magnetic Resonance Imaging 12:75-78 (2000)). These studies indicate that long transmission lines, even without the RF coil, show significant heating, whereas, RF coils without the cable show no heating.
To avoid heating caused by long transmission lines, there have been attempts to wirelessly transmit MRI signals received at RF coils. In most cases, the wireless transmission of signal relies on passive inductive coupling, a process which could lead to excessive sensitivity loss if the magnitude of inductive coupling is small. To improve the detection sensitivity through inductive coupling, a low noise preamplifier is sometimes connected to the RF coil. The preamplifier is used to amplify the received RF signal prior to the wireless transmission to the receiver connected to the MRI scanner. However, most transistor based low-noise amplifiers require a local DC power source. In the case of implanted or catheter coils, it can be difficult, impracticable, and/or expensive to provide a local DC power source without a wire connection.